We have developed an X-chromosome threshold model of genetic transmission. This model has been applied to data on manic-depressive illness, namely for testing the transmission of the illness (Unipolar form (UP) and Bipolar from (BP)), in families of Bipolar proband. The Bipolar form of the illness is compatible with an X-linked transmission whereas UP forms seem to be more heterogeneous from a genetic point of view. However, we have identified in a particular study a UP form transmitted by the same X-locus responsible for the BP manic-depressive illness.